Method of injection molding of thermoplastic resin

Abstract

The invention is to ensure that in injection molding a molten resin having carbon dioxide dissolved therein as a plasticizer, a sufficient amount of carbon dioxide to achieve satisfactory improvement in flowability is dissolved in the molten resin so that the flowability of the molten resin is significantly improved while enabling the production of a non-foamed molded article. Specifically, in the invention, a molten resin having dissolved therein a sufficient amount of carbon dioxide to improve its flowability is filled into a mold cavity while allowing to foam at the flow front of the molten resin, and the resin in the mold cavity is then pressurized to at least a pressure at which it does not foam.

1. A method of injection molding of a thermoplastic resin, comprising:
 filling a mold cavity with a molten resin that preliminarily contains at least 0.2 wt % and not more than 10 wt % of carbon dioxide dissolved therein to lower its melt viscosity, while allowing the molten resin to foam at the flow front thereof; and then  
 pressurizing the resin in the mold cavity to at least a pressure at which the resin does not foam;  
 wherein a thermoplastic resin having an amount of carbon dioxide dissolved in its molten resin at the molding temperature, when carbon dioxide is supplied from a plasticizing cylinder of an injection molding machine to be dissolved in the molten resin, of not more than 0.3 wt %/MPa with respect to the pressure of the supplied carbon dioxide is used.  
 
   
   
     2. The method of injection molding of a thermoplastic resin as recited in  claim 1 , wherein the thermoplastic resin is selected from the group consisting of polycarbonate resin, polystyrene resin, rubber-reinforced polystyrene, modified poly (phenylene ether), crystalline polystyrene, acrylonitrile-butadiene-styrene resin, styrene-butadiene block polymer, poly (methyl methacrylate) resin, and hydrogenated styrene-butadiene block polymer.